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Eur J Cardiothorac Surg 2005;28:80-82
© 2005 Elsevier Science NL
Division of Cardiac Surgery, S. Raffaele University Hospital, Via Olgettina 60, 20132 Milan, Italy
* Tel.: +39 0226 437 109; fax: +39 0226 437 125. (Email: stefano.benussi{at}hsr.it).
Epicardial ablation have played a consistent role in refueling the popularity of atrial fibrillation (AF) surgery in recent years. The palpable perspective of a role of non-sternotomy, beating heart surgical ablation is increasingly stimulating surgeons, companies and some electrophysiologist.
In this context, transmurality of the presently available alternate physical means is one of the most compelling issues. The evidence provided by Bugge and colleagues [1], supporting the superiority of bipolar with respect to unipolar radiofrequency (RF) in epicardial ablation, is therefore timely and welcome.
Interestingly enough, the author's findings are coherent with recently reported clinical experience. Despite the reported good clinical results, there is no clear-cut evidence demonstrating that any presently available unipolar device can predictably yield an epicardial transmural scar. Epicardial microwave ablation on the beating heart at times fails to accomplish electrical isolation even after repeat ablation [2]. Scarce penetration of epicardial unipolar RF ablations was described by Santiago et al., depending upon composition and thickness of the atrial wall [3]. No such data is available for cryoenergy as yet.
The major obstacle to the progression of the lesion through the subendocardial layer is the convective cooling exerted by blood flow through the atrial chamber.
The clamping mechanism of bipolar devices eliminates convective endocardial cooling and compresses the atrial walls thus reducing tissue thickness and improving contact. Prasad demonstrated bipolar RF atrial ablations to be transmural both acutely and after 3 months in pigs [4]. Despite such straightforward experimental data, in the clinical setting, more than one ablation line may be necessary to achieve isolation [5]. To confirm Bugge's findings, in a recent clinical study with a different bipolar RF device we found a complete conduction block in 92% of the pulmonary vein couples after a single ablation and in 100% after one repeat ablation [6].
In summary, a single bipolar RF ablation can be insufficient to provide acute conduction block. But different authors using different devices all come to the same conclusion: by deploying repeat ablations as necessary, bipolar RF allows a 100% acute efficacy in pulmonary veins isolation [5–7].
This is no small thing. In fact, while it is true that incomplete lesions can have some efficacy in the cure of AF, there is growing evidence indicating a direct cause–effect relationship between incomplete ablation lines and clinical failure. In addition, incomplete isolation of the pulmonary veins has been related to automatic left atrial arrhythmias (left atrial tachycardia/flutter) that are generally worse than AF itself since they have a high rate, they are refractory to medical treatment and difficult to ablate [8].
Some of the presently available bipolar RF devices use an algorithm based on the curve describing impedance changes during ablation as an indicator of achieved transmurality. Bugge's study clearly demonstrates that this method is not reliable at least in the context of epicardial beating heart ablation. As a confirmation to this, Gaynor et al., by using a device with a similar impedance feedback algorithm, reported needing an average of about 3 ‘transmural’ ablations to achieve an actual conduction block across the pulmonary veins [7].
Bugge and colleagues must be commended for the strict electrophysiological endpoints adopted in their study. Some would have been satisfied with achieving loss of entrainment at three or four times the baseline pacing threshold. Not surprisingly, atrial entrainment during high output pulmonary vein pacing (25mA) consistently predicted transmurality. Of course, as described, ablation lines that are acutely transmural can reveal gaps during follow up. But the opposite—that non-isolating lines can become complete with time—is unproven as yet and, frankly quite hard to buy.
So, for practical purposes, acute electrical isolation is presently the best target for AF ablation, as far as sinus rhythm is attainable.
An optional surrogate to on-line electrophysiological assessment can possibly be performing a predefined number (2,3) of bipolar RF ablations deemed enough based on prior experiences with electrophysiological validation.
The growing interest of surgeons in the electrophysiological criteria of transmurality is all but accidental. The cardiologists are treating an increasing number of patients with lone AF by means of percutaneous ablation. Their critical points at that appear to be: appendage exclusion, collateral damage, transmurality. Let us analyze what is in the surgeon's reach. Appendage obliteration can be as easy as a stapler shot. Damage to the structures surrounding the heart is very unlikely if you ablate epicardially since the heat (or cold) is directed inward towards the circulating blood. Additionally, a further advantage of bipolar systems is that of virtually abolishing thermal spread. But, being by definition more invasive (skin incisions, general anesthesia) than endovascular procedures, one thing surgery should not compromise/bargain on is electrophysiological efficacy.
AF ablation technology is moving fast. Innovative unipolar epicardial instruments are being tested as well as the clamping version of about all the known sources of energy (cryo, microwave, etc.).
Now that predictable transmurality is available to the surgeon, pulmonary veins isolation is potentially achievable in most patients. The next two parallel challenges of modern lone AF surgery appear to be how to get to performing the connecting lines—between the encirclings and to the mitral annulus—epicardially, in order to accomplish a complete lesion set, and how to get to doing everything through ports.
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